A resistive bolometric detector usually measures the power of an incident radiation in the infrared range. For this purpose, it comprises an absorbing resistive bolometric element which converts the light flow into a heat flow, which causes a temperature rise of said element with respect to a reference temperature. Such a temperature increase then induces a variation of the electric resistance of the absorbing element, causing voltage or current variations thereacross. Such electric variations form the signal delivered by the sensor.
However, the temperature of the bolometric element usually largely depends on its environment, and especially on the temperature of the substrate which comprises the electronic read circuit. To desensitize as much as possible the absorbing element from its environment, and thus to increase the detector sensitivity, the bolometric element is generally thermally insulated from the substrate.
FIG. 1 is a simplified perspective view of an elementary state-of-the-art resistive bolometric detector 10 for infrared detection illustrating this thermal insulation principle. Such an elementary detector, usually called “bolometer”, here in the form of a suspended membrane, is conventionally part of a one- or two-dimensional array of elementary detectors.
Bolometer 10 comprises a thin membrane 12 absorbing the incident radiation, suspended above a substrate-support 14 via two conductive anchoring nails 16 to which it is fixed by two thermal insulation arms 18. Membrane 12 usually comprises an electric insulator layer, such as for example SiO2, SiO, SiN, ZnS or other, which ensures the mechanical stiffness of membrane 12, as well as a metallic electric interconnection layer deposited on the insulating layer.
A thin layer 20 of resistive they lometric material is further deposited at the center of membrane 12 on the metal interconnection layer, especially a layer of semiconductor material, such as polysilicon or amorphous silicon of p or n type, lightly or strongly resistive, or a vanadium oxide (V2O5, VO2) elaborated in a semiconductor phase.
Finally, the substrate-support 14 comprises an electronic circuit integrated on a silicon wafer, usually known as “read circuit”. The read circuit comprises, on the one hand, the elements for stimulating and reading the thermometric element 20 and, on the other hand, the multiplexing components which enable to serialize the signals originating from the different thermometric elements present in the array detector.
In operation, membrane 12 heats up under the effect of an incident electromagnetic radiation and the generated heat power is transmitted to thermometric material layer 20. Periodically, the read circuit arranged in substrate 14 biases membrane 12 by submitting nails 16 to a bias voltage and collects the current flowing through thermometric element 20 to deduce a variation of its resistance, and thus the incident radiation having caused said variation.
The structure and the operation of such a detector being conventional, they will not be explained in further detail, for the sake of brevity. It should however be noted that membrane 12 performs, in additional to the thermal insulation function, three main functions: an antenna function to receive the radiation, a function of conversion of the received electromagnetic power into heat power, and a function of thermometric measurement of the generated heat power. As it is used as an antenna, the dimensions of membrane 12 are accordingly selected to be of the same order of magnitude as the wavelength of the radiation intended to be measured.
Now, in the terahertz range, wavelengths may reach one millimeter, which thus requires a membrane of the same order of magnitude. However, for such dimensions, the thermal mass, the mechanical hold, and the radiative losses of the membrane are such a problem that they eventually adversely affect the detector efficiency.
Thereby, for such a frequency range, the radiation receive function is decoupled from the other functions. The receive function is thus performed by a planar antenna, and the function of converting the electromagnetic power into thermal power is ensured by the resistive load of the antenna. The dimensions of the resistive load conventionally meet the impedance matching conditions, which depend on the geometry of the antenna and on the nature of the layers supporting it, to obtain an optimal conversion. The resistive load is further in thermal contact with a thermometric component for the measurement of the generated thermal power. The assembly then forms a bolometer with an antenna.
In such a configuration, the then lometric element is independent from the antenna and its size no longer depends on the incident wavelength, but rather on factors determining the intrinsic performances of the detector (sensitivity, signal-to-noise ratio, etc.), in accordance with the requirements of the targeted application, for example, active imaging or passive imaging.
Further, in most cases, the incident electromagnetic radiation is not polarized, so that its reception by a single antenna does not enable to capture all of the electromagnetic power. However, a non-polarized radiation may be considered as the superposition of two components linearly polarized in two orthogonal directions, each of these components transporting half of the wave energy. As known per se, an efficient way to capture an incident electromagnetic radiation is to use two crossed bowtie antennas. Bowtie antennas are discussed, for example, in R. PEREZ's thesis, “Contribution à l'analyse théorique et experimentale de radargrwnines GPR:performances des antennes:apports d'une configuration multistatique”, doctoral thesis, Université de Limoges, 2005, which can be consulted on the following site: http://www.unilim.fr/theses/2005/sciences/2005limo0053/perez_r.pdf.
Document U.S. Pat. No. 6,329,655 describes a bolometer with antenna 30, operating in the millimeter range and provided with two crossed bowtie antennas 32, 34, shown in simplified top and cross-section view in FIGS. 2 and 3, respectively.
The principle of bolometer 30 relies on the capacitive coupling performed between antennas 32, 34, arranged on a substrate-support 36, and a resistive load 38, arranged in a suspended membrane 40 and having a thermometric element 42 deposited thereon (FIG. 3).
Resistive load 38, which takes the form of a square layer arranged at the center of antennas 32, 34, indeed has a surface opposite thereto and thus forms a capacitance with the antennas. The radiation detected by antennas 32, 34 is thus transmitted to load 38 by capacitive coupling.
Under the effect of the transmitted radiation, load 38 heats up and transmits the heat thus generated to thermometric element 42, which in turn heats up and sees its electric resistance modified.
While the fact of suspending the thermometric element above the substrate enables it to undergo an electric resistance variation under the effect of the incident radiation, such a variation however remains minute. Indeed, in the context of the elementary bolometric detector of FIG. 1, at 300° K, a variation by 1 K of the observed scene induces a relative variation of the electric resistance of thermometric layer 20 by approximately 0.04%. Indeed, most of the value of the electric resistance of this element is mainly dictated by the direct environment of the membrane. Especially, the substrate influences the temperature of membrane 12 via the thermal conduction through nails 16 and arms 18, which set approximately 70% of the value of the electric resistance of layer 20. Further, the elements surrounding the membrane, such as the substrate and the package of the detector, also emit a thermal radiation, which sets approximately 20% of the value of the electric resistance of layer 20 for an ambient temperature of 300° K. In the best case, less than 10% of the value of the electric resistance, and more generally less than 1% thereof, are set by the incident radiation. The most part of the electric resistance of thermometric material layer 20 being set by elements unrelated to the observed scene, in the absence of specific measure, the detector read dynamics is thus very limited, which makes such a detector very difficult to use.
To overcome this problem, the elementary bolometric detector is associated with a compensation or skimming structure, aiming at removing the non-useful part of the signal originating from the reading of the thermometric element of the membrane.
FIG. 4 is an electric diagram of an infrared bolometric detector 200 of the state of the art comprising such a skimming structure. Detector 200 comprises a two-dimensional array 202 of unit detection elements 204, or “pixels”, each comprising a sensitive bolometer 206 in the form of a membrane suspended above a substrate, for example, the bolometer illustrated in FIG. 1, connected at one of its terminals to a constant voltage “VDET” and at the other to a MOS biasing transistor 208 setting the voltage across bolometer 206 by means of a gate control voltage “GDET”. Pixel 204 also comprises a selection switch 210, connected between MOS transistor 208 and a node “A” provided for each column of array 202, and driven by a control sitmal “SELECT”, enabling to select bolometer 206 for the reading thereof. The two-dimensional assembly of suspended membranes, usually called “retina”, is placed in a tight package in line with a window transparent to the infrared radiation to be detected and in the focal plane of an optical system (not shown). Transistor 208 and switch 210 are usually formed in the substrate under the influence of the membrane of bolometer 206.
Detector 200 also comprises, at the foot of each column of array 202, a skimming structure 212 comprising a reference bolometer 214 identical to bolometers 206 of pixels 204 from an electrothermal viewpoint and made insensitive to the incident radiation originating from the scene to be observed. Reference bolometer 214 for example comprises a suspended membrane identical to that of sensitive bolometers 206 of array 202, made insensitive to the radiation to be detected by means of a screen 216, in which case the electric resistance of the thermometric element of reference bolometer 214 differs from that of sensitive bolometers 206 by the electric resistance variation that the latter undergo due to the incident radiation. This type of structure, optimal from the common mode viewpoint, is however difficult and expensive to manufacture. As a variation, bolometer 214 comprises a stack identical to the stack forming the suspended portion of the membrane of bolometers 206, and thus the suspended membrane of bolometers 206, without the thermal insulation system essentially comprising thermal insulation arms 18, directly formed on top and/or inside of the substrate. In this case, the electric resistance of bolometer 214 is essentially dictated by the substrate temperature, bolometer 214 being said to be “thermalized” to the substrate. Bolometer 214 is connected at one of its terminals to a constant voltage “VSKIM”, and skimming structure 212 further comprises a biasing MOS transistor 218 setting the voltage across bolometer 214 by means of a gate control voltage “GSKIM” and connected between the other terminal of bolometer 214 and node “A”.
Detector 200 also comprises, at the foot of each column of array 202, an integrator 220 of CTIA type (“capacitive transimpeclance amplifier”) for example comprising an amplifier 222 and a capacitance 224 connected between the inverting input and the output of amplifier 222. The inverting terminal and the non-inverting terminal thereof are further respectively connected to node “A” and to a constant voltage “VBUS”. A switch 226, driven by a signal “RAZ”, is also provided in parallel with capacitance 224, for the discharge thereof. The outputs of CTIAs 220 are eventually, for example, connected to respective sample-and-hold devices 228 for the delivery of voltages “Vout” of the CTIAs in multiplexed mode.
Finally, detector 200 comprises a management unit 230 controlling the different previously-described switches. In operation, array 202 is read line by line. To read a line of array 202, switches 210 of the line of pixels 204 are turned on and switches 210 of the other lines are turned off After a phase of discharge of the CTIA capacitors at the foot of the columns, achieved by the turning-on of switches 226 followed by their turning-off, a circuit such as shown in FIG. 5 is thus obtained for each pixel of the line being read. A current “Top” flows in bolometer 206 of the pixel under the effect of its voltage biasing by MOS transistor 208, and a current “Is” flows in bolometer 216 of the skimming structure under the effect of its voltage biasing by MOS transistor 218. The resulting current difference is integrated by CTIA 220 during a predetermined integration period “Tint”. Output voltage “Vout” of CTIA 220 thus is a measurement of the variation of the resistance of bolometer 206 caused by the incident radiation to be detected since the non-useful part of current “Iop” is at least partly compensated for by current “Is” specifically generated to reproduce this non-useful part.
While the skimming structure just described provides satisfactory results in infrared detection, its application to terahertz detection is not satisfactory.
As previously described, the reference bolometers of the skimming structures of infrared detectors of the state of the art are necessarily desensitized to the infrared radiation originating from the scene. Indeed, only the non-useful part of the currents flowing through the sensitive bolometers is to be reproduced, and thus the useful part of said currents, which correspond, in this application, to the infrared radiation to be detected are not to be compensated for.
However, in the context of a terahertz detection, infrared radiation becomes a disturbing element having a non-negligible influence due to its high intensity, infrared radiation most often having an intensity greater by several orders of magnitude than the intensity of terahertz radiation. It is thus necessary to “process” the infrared radiation to detect the terahertz radiation.
However, if a skimming structure similar to that of an infrared detector, that is, desensitized to the radiation originating from the scene, is provided in a terahertz detector, the skimming structure is also desensitized to infrared radiation, so that it does not reproduce the non-useful part corresponding to this radiation. Since it is not reproduced, this non-useful part is thus not compensated for in the current flowing through the sensitive bolometers. Now, as previously described, infrared radiation being greater by several orders of magnitude than terahertz radiation, the useful part of the current of the sensitive bolometers corresponding thereto is thus “drowned” in the non-useful part of the current corresponding to infrared radiation. The detector thus obtained thus finally has a very low sensitivity to terahertz radiation.
Further, a skimming structure which is not desensitized to the radiation originating from the scene cannot be envisaged, since such a structure would reproduce the useful part corresponding to terahertz radiation, which useful part would be compensated for in the current of the sensitive bolometers.